1. Field of the Invention
This invention relates to astronomical body center tracking horizon sensors and more specifically to phase locked electronic circuit loops utilized in conjunction therewith.
2. Description of the Prior Art
Spacecraft, such as earth-orbiting satellites, generally require a sensing means coupled to a measurement device that produces error signals which can be utilized to correct the flight attitude of the spacecraft, to produce antenna-positioning steering signals, and to generate other guidance and timing quantities. For spacecraft which operate in close proximity to radiant astronomical bodies, a body center tracking horizon sensor operating in the infrared spectrum is often used.
A body center tracking horizon sensor is generally composed of an infrared sensor, as the sensing means, a spinning spacecraft, as the scanning means, and an earth center pointing phase locked horizon sensor electronic loop, as the measurement device. An earth center pointing phase locked horizon sensor electronic loop operates, in general, in the following manner. The frequency of an output signal of the earth center pointing phase locked horizon sensor electronic loop, which signal has the lowest subharmonic frequency of an oscillator in the electronic loop, wherein a subharmonic frequency is defined as the fundamental frequency of an oscillator divided by a positive, non-zero integer, N, is matched to the frequency of the signal input from the infrared sensor by applying an appropriate voltage to the oscillator, the magnitude of which depends upon the difference is frequency between the two signals. Errors in frequency normally occur only during the initial operation of a body center tracking horizon sensor. The phase angle of the output signal of an electronic oscillator is matched to that of the signal input from the infrared sensor by aligning the leading edge of the lowest subharmonic frequency output signal of the earth center tracking phase locked horizon sensor circuit to the center of the infrared sensor output signal by applying a voltage signal to the oscillator which is the output of a loop shaping filter, the input of the filter being proportional to the difference in phase angle between the two signals. The difference in time between the crossings of the two horizons of a radiant astronomical body, as sensed by the infrared sensor, is utilized by an earth center tracking logic circuit in conjunction with the lowest subharmonic oscillator output signal to generate the appropriate voltage signals which are applied to the oscillator. Phase angle correction occurs throughout the operation of the body center tracking horizon sensor, after frequency correction has been rendered. Based upon this operation, body center pointing can be accomplished; that is, the precise center of a preselected radiant astronomical body, such as the earth, can be accurately determined. It is apparent that in order to effectively accomplish earth center pointing in the celestial environment, infrared spectra from other than the pre-selected astronomical bodies in the scan path sensed by the body center tracking horizon sensor device must be ignored. Furthermore, relatively large initial input errors must be overcome, so that the device can rapidly acquire the center of the earth for the hereinbefore stated purposes. These initial input errors result from relatively large initial spacecraft spin frequency variations.
Infrared horizon sensors have been designed and built which utilize electronic circuits which feature moon and sun infrared spectra rejection. An example of such a device is described in U.S. Pat. No. 3,920,994 issued to D. R. Cargille on Nov. 18, 1975. Other infrared horizon sensors have been designed and built in which earth center pointing is featured. However, the electronic circuitry of said devices have no infrared spectra rejection capability. Furthermore, such devices are only capable of acquiring the center of the earth for a relatively restricted range of input signal frequency errors, on the order of .+-.10%.
The advantage of the present invention is to combine the capability of accurate earth center pointing with sun and moon infrared spectra rejection and with the capability to pull-in to a phase locked condition despite an initial frequency error on the order of two-to-one. The novelty of the present invention is to simultaneously accomplish these functions.
Accordingly, it is a general purpose of the present invention to provide an earth center pointing phase locked horizon sensor electronic loop operating in the infrared spectra environment.
It is another purpose of the invention to provide a horizon sensor which rejects moon and sun infrared spectra.
Yet another purpose of the invention is to provide an infrared horizon sensor which is capable of acquiring the center of the earth for an initial input error of magnitude on the order of two-to-one.